1. Field of the Invention
This invention relates to a process for manufacturing an electronic circuit device by which solder can be applied to electrodes of electronic parts or circuit boards without using any flux. It also relates to a process for manufacturing an electronic circuit device by which electronic parts having solder balls can be bonded to circuit boards.
2. Description of the Related Art
With regard to the joining of electronic circuit devices, a mounting method making use of flip-chips and a bonding method such as BGA (ball grid array) making use of solder bumps attract notice as electronic instruments and electronic parts are made to have a smaller size, a higher density and a higher performance. Such methods are energetically developed. The bonding method making use of solder bumps is characteristic of an excellent electrical performance exhibited because of wiring length that can be made shorter than that of leads or the like and also an ease of mounting because of bumps arranged in grids so that terminal pitches can be made smaller than QFPs.
In this bonding method, solder bumps must be previously applied to electrodes provided on the side of the board or substrate and must be surely applied to the intended electrodes without any defects such as uneven solder quantity and bridges.
Bumps are applied by, e.g., (1) a method in which solder films are formed by plating or vacuum evaporation, (2) a method in which a solder paste is printed and (3) a method in which solder balls are laid out.
In the method (1), solder films are formed at bonding areas by plating or vacuum evaporation, and a flux is coated thereon, which is thereafter heat-melted to provide bumps.
In the method (2), as reported in, e.g., 1st Symposium xe2x80x9cMicrojoining and Assembly Technology in Electronics ""95xe2x80x9d, Draft Collection, pp. 187-192, a solder paste used in conventional surface packaging is printed on electrodes, followed by heat-melting.
In the method (3), as disclosed in, e.g., U.S. Pat. No. 5,284,287, solder balls are laid out on an aligning jig, namely, a positioning plate having recess (i.e. holes or concaves) formed correspondingly to the electrodes, and are aligned with the electrodes, followed by heat-melting to provide bumps.
When two different materials or members are to be soldered together, a flux or a cream containing flux has been used to remove oxide films formed on the soldered surfaces, to maintain these surfaces clean by preventing oxidization thereof and to promote the wettability of solder on these surfaces. However, when such a flux or flux-containing cream has been used, there has been a tendency for voids to be generated due to the evaporation of the flux during a soldering operation, as illustrated in FIG. 1(c), which results in a lowering of strength and hence reliability of the soldered junction.
In above three types of methods for providing solder bumps, the method (1), in which solder films are formed by plating or vacuum evaporation, has problems that it requires equipment cost for the plating or vacuum evaporation compared with other methods and also makes it difficult to control layer thickness and composition to form uniform solder bumps. The method (2), in which a solder paste is printed, has a problem that when the solder paste is not completely transferred when printed, it remains on a metal mask to make uneven or insufficient the quantity of the solder applied. To solve such a problem of insufficient transfer, a method is proposed in which the printed solder paste is heat-melted while keeping the mask brought into close contact with the substrate, which, however, has a problem that the solder is not transferred if the substrate warps when heated. The method (3), in which solder balls are laid out an aligning jig having holes or concaves formed correspondingly to the electrodes and are mounted on the electrodes followed by heat-melting, can achieve less unevenness in the quantity of solder to be applied than the methods (1) and (2), and hence can achieve less unevenness in the height of bumps.
However, any of the above methods make use of the flux. Accordingly, they require a cleaning step because flux residues are left after the heat-melting. Hitherto, the flux residues have been removed by cleaning with organic solvents such as chlorofluorocarbons and trichloroethane, having an excellent cleanability. These cleaning agents, however, have caused an environmental problem of the ozone-shield destruction. The voids that are produced when the flux is taken into bonding areas have also come into question. In particular, since spherical bodies, like the solder bumps, are arranged in grids, it is difficult to carry out cleaning compared with solder bonding of conventional chip elements and QFPs, so that the flux residues cannot be completely removed to bring about a possibility of causing corrosion and migration.
In the method (2), if the solder is not completely transferred and remains on the metal mask, the surface must be cleaned every time the solder paste is printed. In the method (3), too, if the flux adheres to the aligning jig at the time of mounting, there is a problem that the solder balls are not transferred at the time of next mounting because of a tackiness of the flux. Thus, like the method (2), in which the solder is applied using solder paste, the aligning jig must also be cleaned, and this is a problem in respect of work efficiency and productivity.
In an effort to solve this problem, a low-residual, low-activity flux requiring no post-soldering washing operation has been proposed. However, since such a low-activity flux is easily oxidized when heated in the atmosphere, the soldering operation must be performed in a belt furnace filled with N2. Such a furnace is partitioned by shutters into an inlet gas purge chamber, a heating, melting, and bonding chamber in which air is replaced by N2 sufficiently to maintain the oxygen concentration in this second chamber as low as about 70 ppm, and an outlet gas purge chamber.
Although this bonding method is easy to use, it requires the N2-filled belt furnace. Therefore, fluxless soldering using an ion beam has been proposed in J. Vacuum Science Technology, 20(3), March 1982, pages 359-363. In soldering without flux, the oxide film must be prevented from growing on a solder surface after the surface has been cleaned by sputtering. To this end, the members to be soldered and the soldering material are aligned, and the soldering material is heated in a nonoxidizing atmosphere, which is, unfortunately, impractical in view of the facility needed to perform the operation. More particularly, a large, complicated facility is required, which inevitably results in technical difficulties in the alignment of the members.
Another fluxless soldering method has been proposed in Japanese Kokai (P) 58-3238, which describes the juxtaposition of two members to be bonded in a vacuum chamber, wherein the solder surfaces are cleaned by ion irradiation, aligned by overlapping, and then irradiated again with an ion beam to melt the solder. Since the cleaning, aligning, and heating operations are all performed within the vacuum chamber, workability and productivity are very low. More particularly, the method requires an alignment device that picks up at least one of the two members to be bonded, turns it over, carries it to the other member, and aligns a number of bonding portions on the two members. This operation requires a large-sized vacuum chamber, which increases the cost of the entire apparatus while decreasing its efficiency. Moreover, the possibility of contamination of the vacuum chamber is very high, and there is a limit to the thermal capacity of the apparatus due to the use of an ion beam, causing the simultaneous heating of large substrates to be difficult.
Another example is disclosed in Japanese Kokai (P) 3-171643. In this method, an atom or ion beam irradiation device and a post-processing device for aligning bonding portions and heating solder are separately provided. The interior of the post-processing device is filled with an inert gas. Members to be bonded together are aligned in a plenum chamber of the post-processing device and heated to a temperature below the melting point of the solder, under pressure, to temporarily fix the members, which are then transported to a heating/melting chamber of the post-processing device for the soldering to be performed.
Although no aligning device is required according to this method, the post-processing device must be kept at vacuum pressure, and the alignment and soldering performed in the post-processing device.
In spite of the problems of the prior art, the proliferation and advancement of computers and other apparatus that require electronic circuit devices employing flip-chip connections (wherein a number of fine connections are made using fine solder balls) requires a similar advancement in the precision of such connections. However, it has been very difficult to bond a number of parts using flip-chip connections within an inert atmosphere in a vacuum chamber with high precision. Furthermore, the transportation of such parts into, out of, and within the vacuum chamber, together with the evacuation of the vacuum chamber, etc., have proven troublesome, causing the workability of the prior art systems to be low.
Accordingly, a first object of the present invention is to provide a process for manufacturing an electronic circuit device by which solder can be applied at a low cost by fluxless bonding that enables easy and simple application of uniform solder and requires no cleaning step.
The above first object can be achieved by a process for manufacturing an electronic circuit device by applying a solder material to an electrode the electronic circuit device has; the process comprising the steps of;
removing an initial surface oxide film and/or an organic contaminant film from the surface(s) of the solder material and/or electrode;
covering the solder material and/or the electrode with a liquid; and
heat-melting the solder material;
to carry out solder bonding without using any flux.
The object can also be achieved by a process for manufacturing an electronic circuit device by applying a solder material to an electrode the electronic circuit device has; the process comprising the steps of;
printing or coating on the electrode a paste comprising a mixture of a liquid and a solder powder; the liquid having a reducing action to remove an initial surface oxide film and/or an organic contaminant film from the surface(s) of the solder material and/or electrode; and
heat-melting the paste;
to carry out solder bonding without using any flux.
The object can still also be achieved by a process comprising the steps of;
mounting a solder material on an electrode with registration; the solder material being a material on which an initial surface oxide film has been thinly formed, and the electrode being an electrode on the surface of which an oxidation preventive film has been formed;
covering the solder material and/or the electrode with a liquid; and
heat-melting the solder material; to carry out solder bonding without using any flux.
The object can still also be achieved by;
mounting an anisotropic conductive film on an electrode with registration; the film being a film on which solder has been formed correspondingly to an electrode to which the solder is to be applied, and the electrode being coated with a liquid; and
heat-melting the solder material.
Meanwhile, after the initial oxide films are physically removed and the solder balls are laid out on the jig and mounted on the electrodes, the solder balls may be heat-melted in a non-oxidizing atmosphere. This enables prevention of the reoxidation and makes it unnecessary to carry out not only the cleaning but also the coating of the liquid. The problems previously discussed may occur also when electronic parts having solder balls are bonded to circuit boards.
Accordingly, a second object of the present invention is to provide a process for manufacturing an electronic circuit device by bonding electronic parts to circuit boards in a high reliability by fluxless bonding.
The above second object can be achieved by a process for manufacturing an electronic circuit device by bonding an electronic part having solder balls to a circuit board; the process comprising the steps of;
removing an initial surface oxide film and/or an organic contaminant film from the solder material and/or the circuit board at its area to which solder is to be applied;
covering with a liquid the solder material and/or the circuit board at its area to which solder is to be applied; and
heat-melting the solder material; to carry out solder bonding without using any flux.
The object can also be achieved by a process comprising the steps of;
mounting a solder material with registration, on a circuit board at its area to which solder is to be applied; the solder material being a material an electronic part has and on which an initial surface oxide film has been thinly formed, and the area to which solder is to be applied being an area on the surface of which an oxidation preventive film has been formed;
covering with a liquid: the solder material and/or the area to which solder is to be applied; and
heat-melting the solder material; to carry out solder bonding without using any flux.